Ergothioneine

Ergothioneine is a sulphur-containing amino acid derivative produced almost exclusively by mushrooms and certain bacteria — humans can't synthesise it but our bodies have a dedicated transporter (OCTN1, encoded by SLC22A4) that actively pulls it into cells, including skin keratinocytes. It accumulates inside mitochondria and acts as an antioxidant exactly where free radicals are generated. Unlike vitamin C, ergothioneine is stable across the full physiological pH range, doesn't oxidise on storage, and has no known toxicity even at very high doses. It scavenges hydroxyl radicals (the most damaging free radical class) more efficiently than glutathione and protects mitochondrial DNA from oxidative damage that drives skin aging. The Ordinary launched a dedicated 0.5% serum in 2024; Estée Lauder, Sulwhasoo, and several K-beauty brands now feature it in premium serums. It's expensive to manufacture (commercial-scale fermentation only solved in the last decade), which is why concentration claims matter — sub-0.1% formulations are vanity inclusions.